This invention relates to a method of forming a tapered or pointed end on an extruded thermoplastic resin molding strip. More particularly, this invention relates to a novel method of forming a flexible tapered end on a plastic molding strip of the type which may be mounted on a side panel or door of an automotive vehicle for protection and decoration.
Before this invention a tapered end was formed on an extruded thermoplastic strip by removing a V-shaped notch from the end of the strip as viewed from the top. Opposing triangular shaped legs were thereby formed on either side of the notch, extending from the apex of the notch to the end of the strip. The depth of the notch corresponded to a desired degree of taper on the end. The surfaces of the legs adjacent to the notch were heated from the apex along their entire length to melt the plastic. The legs were then pressed together to fuse the plastic and held in place until it solidified.
A recurrent problem with strips formed in the above manner has been their tendency to arch at the tapered end. This arching is probably due to the residual stress created in the plastic by the tapering operation. When such a strip is applied to a substratum with an adhesive, this residual stress may cause the end to pull loose, and once exposed, the entire strip may delaminate.
It is therefore an object of this invention to provide a method of forming a tapered end on an extruded thermoplastic resin molding strip so that it tends not to pull away from a substratum when applied with a suitable adhesive. It is a further object of my method to provide a tapered end with flexibility and wherein the stress brought about by conventional tapering methods is essentially relieved.
In accordance with a preferred practice of my invention, these and other objects may be accomplished as follows. A molding strip is formed having a top surface shaped to provide desired decorative and/or protective functions. The top surface typically comprises a plastic crown portion along the longitudinal center, parallel to the bottom, and decorative foil covered edges set back from the crown. The crown is adapted to a absorb minor impacts to protect the substratum and the decorative foil edges of the strip. The bottom of the strip preferably defines a generally flat plane to accommodate bonding to a substratum such as an automotive vehicle door or fender with a suitable adhesive. The strips are usually made by extruding long lengths of a suitable thermoplastic resin such as poly(vinyl chloride) (PVC) through a die, and cutting the strip into segments of desired lengths. In many instances the appearance of a trim strip is greatly enhanced when a pointed taper is formed at its ends. The subject invention involves an improved method of forming these tapered ends so that they do not tend to thereafter delaminate from the substrate to which they are attached.
Like the prior art, my method entails first removing a V-shaped wedge from an end of a strip shaped like that described above. The apex of the V is located at a point remote from the end of the strip on the central longitudinal axis of the strip. The distance between this point and the end of the strip determines the degree of taper. Removal of the V-shaped wedge creates two like triangular legs which are later drawn together to create the pointed taper. The remaining steps of my method depart from the prior method and provide a pointed tapered end with unexpected flexibility and resistance to deformation due to residual stress.
In a preferred embodiment, the legs at the notched end are bent upwardly about 45.degree. by any suitable means. This bending step stretches the plastic at the backside or bottom of the strip along the length of the notch and is believed to counteract the tendency of the end to arch after the taper is formed.
While the notched end of the strip is maintained in the bent state, the edges of the legs adjacent the V-notch are heated at the ends remote from the apex to melt the surface of the plastic. The edges are then pressed together and the plastic is allowed to solidify, fusing the legs together at the point of the taper. Joining the edges only near the point is believed to create less residual stress in the taper than the prior art method of bonding along the entire length of the notch.
After the edges are joined, the end is released from the bending means and it returns substantially to the unbent state. The operation is then completed by forming a series of cuts into the back of the strip perpendicular to the longitudinal axis. The cuts are made at about 1/8 inch intervals from between the apex point of the closed notch and the pointed end of the taper. The cuts penetrate through the bottom of the strip to just below the top surface. They provide the end with flexibility and probably provide a release for any latent residual stress brought about by joining the edges near the pointed end.